The present invention relates to thin film electrical components, and particularly, to a flexible, micro-miniature, waterproof, biocompatible, thin film circuit package that can be patterned to function as a connector, sensor, or other electrical component. More particularly, the present invention relates to improvements in handling and configuring thin film circuit packages during manufacture.
A thin film is typically a film of 0.01 .mu.m to 0.5 .mu.m deposited on a glass, ceramic, or semiconductor substrate to form, for example, a capacitor, resistor, coil, or other circuit component. Thin film integrated circuits consist entirely of thin films deposited in a patterned relationship on a substrate.
One aim of the present invention is to develop an improved biocompatible thin film sensor or circuit suitable for use in the human body. It has been observed that satisfactory biocompatible thin film sensors or circuits cannot generally be produced by relying exclusively on the materials and techniques used to make conventional thin film structures.
Testing and monitoring of many biological conditions in the human body or other ionic liquid environments would be enhanced by the development of a waterproof thin film biosensor of miniature size and flexible character. However, conventional thin film structures are not generally designed to be biocompatible with the human body and are made using processes which are not suitable for use with biomaterials. Also, conventional thin film structures are not flexible.
A silicon wafer typically provides the foundation, platform, or carrier plate on which a conventional thin film structure is built. Automatic handling equipment is used to move the silicon wafer between manufacturing stations as a thin film structure is developed layer-by-layer on the silicon wafer. In this invention, the first thin film layer bonded to the silicon wafer carrier plate is used as a substrate of the thin film structure. Once the thin film structure is constructed, the silicon wafer carrier plate is released from attachment to the substrate of the thin film structure and discarded.
A thin film structure can be damaged during the release of the thin film structure from its silicon wafer carrier plate if the structure is made of any material which is incompatible with the acid etching solution or other release agent used to break the bonds coupling the substrate of the thin film structure to its silicon wafer carrier plate. It has been observed that many biomaterials of the type that could be used in the construction of a biologically compatible thin film structure are damaged or lose function upon exposure to conventional silicon wafer-releasing agents. The etching solutions normally used to dissolve a silicon wafer carrier plate are not compatible with biomaterials contained in a biocompatible thin film structure.
It has also been observed that thin film metal conductors deposited on a polymer substrate coupled to a carrier plate tend to fracture and delaminate upon release of the polymer substrate from its carrier plate foundation. It is thought that any difference in the coefficients of thermal expansion of the polymer substrate and its carrier plate foundation leads to the development of internal stresses in the polymer substrate as it is heat-cured, because its coefficient of thermal expansion is different from the carrier plate. Thin film structure fracture and delamination problems can result unless careful attention is given to the thermal expansion coefficients of the carrier plate foundation and substrate materials used to construct the thin film structure.
Conventional thin film structures are known to contain an adhesion metal film to enhance bonding of a noble metal film to its underlying polymer substrate. Such an adhesion metal film layer is a useful bonding tool because noble metals used to form electrical circuits in thin film structures do not adhere well directly to polymer substrates. For that reason, the adhesion metal layer is situated between the polymer substrate and the noble metal film layer.
It has been observed that thin film structures suffer performance losses because of interdiffusion of abutting adjacent adhesion and noble metal film layers during any high-temperature processing of the thin film structure, for example, during heat-curing of a polymer precursor solution to provide a polymer insulation layer on the metal film layers. Performance losses include, for example, loss of flexibility of the thin film structure itself and development of metal film layer adhesion problems. In addition to these mechanical problems, metal interdiffusion can cause changes to occur in the electrical properties of the noble metal layer.
The common solution to the metal interdiffusion problem in the semiconductor industry is the addition of a refractory metal layer (e.g., tungsten or tantalum) between the adhesion and noble metal film layers. This extra layer acts as an interdiffusion barrier. Processing of a thin film structure, however, is severely complicated by the deposition and patterning of such an additional refractory metal film layer.
One object of the present invention is to provide a biologically compatible thin film electrical component suitable for use in the human body or any other ionic liquid environment.
Another object of the present invention is to provide a thin film electrical component configured and patterned to function as a biosensor includable in a medical device implanted in a human body and a method of making this thin film electrical component to preserve its biosensor function capability.
Still another object of the present invention is to provide a thin film structure which can be fabricated without the occurrence of stress damage at the release of the thin film structure substrate from its carrier plate foundation.
Yet another object of the present invention is to provide a thin film structure that can be insulated without interdiffusion of metal film layers contained in the thin film structure.
According to the present invention, a thin film electrical component includes a rigid glass carrier plate, a substrate bonded to the rigid glass carrier plate, and means for providing an electrical circuit. The substrate comprises a polymer establishing a bond with the rigid glass carrier plate that is broken upon immersion of the substrate and the rigid glass carrier plate in either a boiling water bath or a room temperature physiologic saline bath to release the substrate from attachment to the rigid glass carrier plate. The means for providing an electrical circuit is bonded to the substrate and undisrupted during release of the substrate attachment to the rigid glass carrier plate.
One feature of the present invention is the provision of a thin film electrical component in which the polymer substrate can be released from its carrier plate without the use of any release agent or technique that could damage biomaterials contained in the substrate or means for providing an electrical circuit formed on the substrate. The selection of the proper glass for the carrier plate and the proper polymer for the substrate is critical. In particular, a polymer that develops a bond to glass that is strong enough only to withstand circuit fabrication processes but may be broken with biocompatilbe boiling water or biocompatible warm saline exposure is an important feature of the invention. Advantageously, the use of non-biocompatible silicon-etching release agents is avoided.
Those skilled in the art will recognize that an unmodified PMDA-ODA type polyimide is generally considered to be inferior compared to other possible substrate materials because of its marginal adhesive strength when bonded to most carrier plate foundations. Thus, for most thin film applications, adhesion-promoting agents are added to it so that this polyimide is modified to improve its bonding capability. Nevertheless, in the present invention, marginal adhesive strength is desirable to permit the glass carrier plate to be released from the polyimide substrate using biocompatible releasing agents and techniques. Thus, in the preferred embodiment, the polymer substrate comprises an unmodified PMDA-ODA type polyimide.
In preferred embodiments, the rigid glass carrier plate comprises a low-expansion type glass having a characteristic coefficient of thermal expansion and the polymer forming the substrate comprises a polyimide having a coefficient of thermal expansion that is substantially equivalent to the coefficient of thermal expansion of the low-expansion glass. Such a match in thermal expansion coefficients will cause the rigid glass carrier plate and the polymer substrate carrying the means for providing an electrical circuit to expand at about the same rate during exposure of the thin film structure to an elevated temperature. Advantageously, separation of the means for providing an electrical circuit from the substrate is avoided during release of the polymer substrate from attachment to the rigid glass carrier plate.
Accordingly, another feature of the present invention is that the carrier plate glass and the substrate polymer are carefully selected so that their coefficients of thermal expansion are about the same. Advantageously, thermal matching of this type reduces the likelihood that serious fracture and delamination problems will arise during release of the polymer substrate from the glass carrier plate.
Yet another feature of the present invention is the provision of a polymer insulation layer with a relatively low cure temperature. Without metal interdiffusion considerations, a logical choice for this insulation layer might be a photoimageable polyimide because the layer can be patterned without the use of additional photoresist steps. Unfortunately, currently available photoimageable polyimides require a high-cure temperature (450.degree. C.) to drive out the photosensitizers. In the preferred embodiment, a BTDA-ODA polyimide is used, with positive photoresist, to form the patterned insulation coating. This material can be cured as low as 250.degree. C., causing significantly less interdiffusion and resulting problems.
In the preferred embodiment, one type of polyimide is used to provide the polymer substrate and a different type of polyimide is used to provide the polymer insulator. The selection of the "substrate" polyimide is governed by its releasability and thermal-expansion compatibility with a glass carrier plate. For example, either a PMDA-ODA or BPDA-PPD type polyimide is suitable. On the other hand, the selection of the "insulator" polyimide is governed by the magnitude of its cure temperature compared to the minimum temperature at which metals in the thin film electrical component begin to interdiffuse and by its adhesive strength to the underlying polymer and metal layer. For example, as noted above, a BTDA-ODA polyimide is suitable.
Also according to the present invention, a method is provided of making a thin film electrical component. The method comprises the steps of providing a rigid glass carrier plate having a flat surface, coating the flat surface with a first polyamic acid precursor solution, and curing the first polyamic acid precursor solution using heat at a first temperature to provide a layer of polyimide film bonded to the flat surface. In a preferred embodiment of the present invention, the polyimide film has a coefficient of thermal expansion that is substantially equivalent to the coefficient of thermal expansion of the rigid glass carrier plate so that creation of internal stresses in the polyimide film during curing is avoided. In certain other embodiments, glass carrier plates and polyimide films having substantially different coefficients of thermal expansion can be used.
Preferably, the coating step further comprises the steps of dispensing the first polyamic acid precursor solution onto the flat surface of the rigid glass carrier plate, and spinning the rigid glass carrier plate about an axis orthogonal to the flat surface to create a smooth polyamic acid precursor solution coating of substantially uniform thickness across the flat surface. This is a conventional spin-coating process commonly used in the semiconductor industry. The rigid glass carrier plate is spun at a speed between 1,000 and 6,000 revolutions per minute during the spinning step.
The method further includes the steps of forming means for providing an electrical circuit on the polyimide film, and exposing the rigid glass carrier plate, the polyimide film, and the means for providing an electrical circuit to either a hot water bath or a body temperature physiologic saline bath for a period of time sufficient to break the bond connecting the polyimide film to the rigid glass carrier plate. The warm saline bath provides a biocompatible releasing agent which operates to cause the polyimide film to be released from the rigid glass carrier plate without separating the means for providing an electrical circuit from the polyimide film.
An electrical circuit is provided in the thin film electrical component by depositing a first adhesive metal layer on the polyimide film, a noble metal layer on the first adhesive metal layer, and a second adhesive metal layer on the noble metal layer to sandwich the noble metal layer between the first and second adhesive metal layers, and patterning the deposited noble metal layer and first and second adhesive metal layers to define means on the polyimide film for providing an electrical circuit. Next, a second polyamic acid precursor solution is provided on the means for providing an electrical circuit by dispensing the second polyamic acid precursor solution onto the means for providing an electrical circuit, and spinning an assembly comprising the rigid glass carrier plate, the polyimide film, the means for providing an electrical circuit about an axis orthogonal to the flat surface of the rigid glass carrier plate. The assembly is spun to create a coating of second polyamic acid precursor solution on predetermined exposed portions of the polyimide film and the means for providing an electrical circuit.
The second polyamic acid precursor solution is then cured using heat at a second temperature to provide a polyimide insulation coating on the means for providing an electrical circuit. In cases where interdiffusion between noble and adhesive metal layers is a problem, the second temperature is less than a characteristic minimum temperature at which interdiffusion of the noble metal layer and the first and second adhesive metal layers occurs so that substantial interdiffusion of the noble metal layer and the first and second adhesive metal layers is avoided during heat curing of the second polyamic acid precursor solution.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of a preferred embodiment exemplifying the best mode of carrying out the invention as presently perceived.